EM Nerd-The Case of the Bridge to Nowhere

The use of mechanical cardiopulmonary resuscitation (mCPR) has been a polarizing topic almost from its inception. The idea, a simple one. Why not build a machine that is capable of doing perfect chest compressions? A simple, effective way to eliminate the inconsistencies common to manual chest compressions. And yet, despite building a number of devices that are capable of performing almost perfect, consistent chest compressions, the evidence supporting their use has been universally negative.

In a December 2016 letter published in Circulation, Buckler et al queried the Cardiac Arrest Registry to Enhance Survival (CARES) registry from January 2013 to December 2015 and examined the outcomes of out-of-hospital cardiac arrest (OHCA) patients who did and did not receive mCPR (1). The authors included 80,861 patients in their final analysis. As with any non-randomized data set there were obvious and potentially confounding differences between the two groups. Patients receiving mCPR were more likely to have an unwitnessed arrest (57.3% vs 55.7%), an automated external defibrillator placed (33.3% vs 28.3%), an advanced airway placed (87.4% vs 79.0%), an impedance threshold device used (41.8% vs 13.4%), and undergone pre-hospital targeted temperature management (16.6% vs 12.2%). The authors also noted that although the overall deployment of mCPR rose over the 2-year period from 20.6% to 23.4%, its utilization at individual sites varied widely.

The authors reported that patients who received mCPR fared markedly worse than those who received manual compressions. Survival to hospital discharge was 11.3% and 7.0% in the manual and mCPR groups respectively. Survival with good neurological outcome again favored the patients who received manual CPR (9.5% vs 5.6%). Even when controlling for the multiple potential confounders through logistical regression and sensitivity analyses, the authors reported the use of mCPR was associated with significantly worse outcomes.

This results are concerning. The question now becomes with such consistently negative data, why do some continue to advocate their use? In a large degree the cognitive dissonance surrounding these devices is in a large part due to the literature on mCPR flies directly in the face of everything we are taught regarding the importance of chest compressions in OHCA. Why has a tool that has been proven to perform chest compressions far superior to its human counterparts, failed to demonstrate improvements in clinically meaningful outcomes? Now not even I have fallen so far down the nihilistic rabbit hole to assert the futility of chest compressions in cardiac arrest. The problem does not lie in the mechanical devices, nor in the the evidence examining them. Rather, the fault is in our interpretation of this literature due to a subtle but common misunderstanding of the benefits of CPR in OHCA.

Most modern discussions regarding the importance of chest compressions focus around the coronary perfusion pressure (CPP) generated by the rhythmic compression of the thoracic cavity. A study by Parades et al is widely cited, in which the authors directly measured the CPP in patients arriving to the Emergency Department in cardiac arrest (2). The patients were monitored using central arterial and venous cannulation. Using the arterial and venous pressures, the authors estimated the CPP in each patient. The authors reported that it was not until the patients achieved a CPP of 15 mm Hg or greater that ROSC was reliably achieved. The clinical leap of faith is by performing continual high quality chest compressions, a higher CPP is achieved which will lead to improved survival. These are the physiological underpinnings behind high-performance CPR.

This concept was demonstrated in a clinical context when Bobrow et al published their findings on the use of early continuous chest compressions for the treatment of OHCA. Published in JAMA in 2008, the authors reported in a before and after fashion on the introduction of continuous chest compressions, early defibrillation, and minimal ventilations in their EMS system in Arizona (3). When comparing patients who received continuous CPR vs traditional CPR, the authors found a significant improvement in overall survival (5.4% vs 1.8%) and an impressively high improvement in survival in patients who had a shockable rhythm (17.6% vs 4.7%). Since Bobrow’s publication a number of additional authors have published on their successful transition to a similar management strategy.

In 2015, the AHA definedthe measures that constitute high quality CPR, citing a preferred rate of between 100-120 compressions per minute (4). These guidelines were based on a 2015 article by Idris et al published in Critical Care Medicine in which the authors performed a secondary analysis of the ROC-PRIME trial and determined the chest compression rate for optimal survival (5). And while this data does appear to support the benefits of high-performance CPR, RCT data has been far less positive.

When Jost et al conducted an RCT comparing standard CPR to a protocol that optimized CPR performance, the authors found no difference in survival (5). When Nichols et al published the results of their 26,148 patient RCT comparing continuous chest compressions to traditional CPR, that authors found no survival benefit (7). All 5 RCTs comparing standard manual CPR to mCPR failed to demonstrate a benefit in patient outcomes (8,9,10,11,12). Even the analysis by Idris et al, utilized by the AHA to support their recommendations on “optimal” rate, was a secondary analysis of data taken during the first five minutes after EMS arrival and required multiple statistical manipulations to find a minimal important benefit (5). In fact, virtually no high-quality RCT has ever found a survival benefit to simply doing “better CPR”.

And so why do we see time and time again, strategies to logistically optimize CPR fail to improve clinically important outcomes? I suggest it is simply a failure in the understanding of the true benefits which chest compressions provide. A recent article published in Circulation by Rajan et al serves as a reminder of these benefits. Using the Danish cardiac arrest registry, the authors identified all cardiac arrests from 2005 to 2011 and identified the time from dispatch call to arrival of ambulance to the patient (13). Using this as the proxy for down time prior to defibrillation, the authors examined the benefits of bystander CPR. A total of 7,623 patients met the authors entry criteria. Approximately half the cohort were witnessed arrests, and around 30% received bystander CPR. As one would expect the rate of shockable rhythm decreased with increased time from call to arrival of EMS. This decline was far more impressive in the patients who did not receive bystander CPR.

The rates of 30-day survival in patients who received bystander CPR for 5 minutes, 6-10 minutes, 11-15 minutes and >15 minutes prior to EMS arrival were 22.6% (192/848), 15.3% (180/1180), 6.7% (44/613) and 4.7%. In patients who did not receive bystander CPR, the corresponding 30-day survival rates for patients without bystander CPR were 6.7% (111/1617), 3.4% (64/1912), 1.3% (10/792) and 2.7% (9/331), respectively. The authors also noted that patients who did not receive bystander CPR were more likely to be diagnosed with an anoxic brain injury (12.7% vs. 7.3%, p=0.02).

The authors performed multiple logistic regressions and subset analyses based on age, sex, location of arrest (private home vs. public), witnessed status, comorbidities and year of arrest that demonstrated similar findings to their crude analysis. Patients who received bystander CPR did far better than those who did not. These benefits were time dependent and all but disappeared when down time was longer than 20 minutes. In the cohort termed as “best case scenarios” (<65, with a witnessed arrest, and no co-morbidities) the 30-day survival in patients with a minimal response time (<5 minutes) was 54.2% and 30.2%, respectively. By 10 minutes, these rates decreased to 33.1% and 12.2%. By 15 minutes, the chances were 21.0% and 8.1%.

The reason the mCPR trials are universally negative is the very same reason the majority of RCTs examining “better” CPR have consistently failed to demonstrate a clinically important benefit. CPR is a bridge to a definitive intervention. In the case of OHCA, this intervention has traditionally been rapid defibrillation. Bystander CPR has consistently proven to extend the period in which patients will be found in a shockable rhythm and respond to defibrillation. But if the mCPR device arrives at the same moment as the defibrillator, it offers no benefit over rapid defibrillation and in fact, in cases where the logistical complexity of applying these devices delays time to defibrillation, may even be harmful. Chest compressions are a bridge to a definitive intervention. But they are a bridge that rapidly becomes ineffective if no such intervention is applied. And yes mCPR may be a better bridge, but without an alternative destination when defibrillation fails or is not an option, it will collapse just as predictably as manual CPR. To expect mCPR to provide a survival benefit when compared to manual CPR without offering any novel forms of definitive therapy is simply a misjudgment in the benefits chest compressions provide.

The entirety of the literature examining the efficacy of mCPRhas examined its utilization in an unfiltered population of patients who experienced OHCA, randomized at first contact by EMS. But this is not the population in which most advocates of mCPR support its use. There is no question mCPR is not superior to a well trained EMS crew performing manual compressions and rapid defibrillation in the early management of OHCA. What about for the patients who are refractory to our traditional resuscitative efforts?

It is in this subset of patients in prolonged arrest that mCPR may prove to be beneficial. Not because its provides “better” CPR. Not because its augmented hemodynamic support will lead to spontaneous ROSC, (the current literature demonstrated this to not be the case). But because it becomes logistically difficult to maintain high quality compressions while transporting a patient to the Emergency Department, or taking a patient to the cath lab, or cannulating a patient for extracorporeal support. In these cases, mCPR offers theoretical logistical advantages over manual compressions. There is no evidence that these theoretical benefits are in fact clinical realities, but conversely the negative findings offered up by mCPR detractors in no way address this specific question.

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Steven Smith

As someone who works in the prehospital setting in a flight setting, I can assure you that the logistical nightmare of trying to CPR in an aircraft is eliminated with the advent of mCPR. As such, it use to be common place and taught that “patients in cardiac arrest can not go by aircraft due to need for CPR”, but now it is very routine for us to take patients in cardiac arrest with mCPR in place to higher levels of care where they benefit from that rapid transport to definitive care. For example, a patient presented multiple shocks witnessed cardiac arrest and every shock would bring the patient back for a few minutes before relapsing into v-fib. This was continued from prehospital to ER where patient was then found to have a STEMI and warranted air medical transport with mCPR in place (which would have been a 1.5 hrs trip by ground turned into 17min flight). Patient survived 100% neuro intact and discharged 7 days later. This was a real patient and continues to prove its not the fact that mCPR is somehow better than CPR, its the fact mCPR can be done when CPR can not.

I’d like to know your take on Zeiner’s mCPR study (http://dx.doi.org/10.1016/j.resuscitation.2015.07.051). It appears that patients in this (smaller) study who received mCPR were more likely to have favourable characteristics (younger, witnessed arrest, shockable rhythm, bystander CPR) but still had worse outcomes than those receiving human CPR (hCPR)

Nonetheless, I agree that patient selection is paramount, and something we (EMS) need to be better at. Some people should not be resuscitated; some people should; and some should get mCPR to a centre that can provide further care (after a trial of standard resus)

With a more nuanced approach to how and who we resuscitate, we may be able demonstrate those benefits that we all intuitively believe should exist.

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Ken Reed

While I appreciate the level of examination and the conclusions, I wonder if the length of CPR was considered. Transported or sustained duration arrest patients on mechanical CPR would have worse outcomes than those who had shorter periods of manual CPR with ROSC. The other consideration is one of sufficient manpower to maintain a “pool” of rested compressors to maintain high quality chest compressions vs. as rural crew of 2 or 3 providers. The devices may have situational advantages not reflected in the studies.

It is hard to piece this out in the analysis by Buckler et al, but certainly this would have been accounted for in the RCTs as these differences should have been randomized equally between groups. But yes looking at this specific patient population as a potential benefit of mCPR would be a very interesting study.

[…] and observational in nature. However, it was well done and has been reviewed by REBEL EM and EMNerd. Experts, such as Scott Weingart, continue to advocate for its use and emphasize that it is a […]